Page 107 - Chemistry ICSE Class IX

Atomic Structure and Chemical Bonding 95
with the lowest energy is lled rst and then the orbits of increasing
energies are lled.
ƒ Then write the numerals denoting the number of electrons in various
shells starting with the lowest. Each numeral is separated from the
other by a comma.
For example, lithium (atomic number 3) has three electrons in its atom.
2
According to the 2n rule:
The shell corresponding to n = 1 (K-shell) will have 2 electrons and the
shell corresponding to n = 2 (L-shell) will have 1 electron. Therefore, the
electronic con guration of lithium is 2, 1. Electronic con guration may also
be shown through a diagram. For lithium (con guration 2, 1), it is shown
in Fig. 4.12.
The distribution of electrons in various orbits and the electronic
con gurations in the notation form for the rst twenty elements are
presented in Table 4.2.

Table 4.2 The electronic configurations of the first twenty elements
Electronic distribution in
Atomic various shells Electronic
number Name and symbol (n = 1) (n = 2) (n = 3) (n = 4) EQPſIWTCVKQP Valency Nature
of the element
(notation
(Z)
K L M N form)
1 Hydrogen (H) 1 1 +1, –1 Nonmetal
2 Helium (He) 2 2 0 Noble gas
3 Lithium (Li) 2 1 2, 1 +1 Metal

4 Beryllium (Be) 2 2 2, 2 +2 Metal
5 Boron (B) 2 3 2, 3 +3 Metalloid
6 Carbon (C) 2 4 2, 4 +4 Nonmetal (solid)
7 Nitrogen (N) 2 5 2, 5 +1, +2, +3, +4, +5, –3 Nonmetal (gas)
8 Oxygen (O) 2 6 2, 6 –2 Nonmetal (gas)

9 Fluorine (F) 2 7 2, 7 –1 Nonmetal (gas)
10 Neon (Ne) 2 8 2, 8 0 Noble gas
11 Sodium (Na) 2 8 1 2, 8, 1 +1 Metal
12 Magnesium (Mg) 2 8 2 2, 8, 2 +2 Metal

13 Aluminium (Al) 2 8 3 2, 8, 3 +3 Metal
14 Silicon (Si) 2 8 4 2, 8, 4 +4 Metalloid
15 Phosphorus (P) 2 8 5 2, 8, 5 +3, +5 Nonmetal (solid)
16 Sulphur (S) 2 8 6 2, 8, 6 +2, +4, +6 Nonmetal (solid)
17 Chlorine (Cl) 2 8 7 2, 8, 7 –1 Nonmetal (gas)

18 Argon (Ar) 2 8 8 2, 8, 8 0 Noble gas
19 Potassium (K) 2 8 8 1 2, 8, 8, 1 +1 Metal
20 Calcium (Ca) 2 8 8 2 2, 8, 8, 2 +2 Metal

Electronic con gurations of some elements in pictorial form are shown in Fig. 4.13. The number in
brackets is the atomic number of the respective element. The numbers in the innermost circle show the
composition of the nucleus.